The Black Sabbath

 

The Black Sabbath Shu"t Chasam Sofer (O.Ch. 159) writes, "There is great mourning, especially in the lands of Ashkenaz. They are מחמיר on the Shabbos before שבועות like we are מחמיר the Shabbos before תשעה באב." In Ashkenaz Kehillos,  the Shabbos before שבועות was called "Der Schwartze Shabbos" They said Yizkor for the murdered Yidden going back to גזירת תתנ"ו and for all the killings of since then. The אב הרחמים was originally  said only once a year on the  Schwartze Shabbos.  Eventually, they added Shabbos Chazon for אב הרחמים to be said.  Hence, Minhag Ashkenaz was to say אב הרחמים only twice a year.